Friends teacher include speaker in context

[chiehminwei]

Patch description

• Add a command line option --include-speaker-in-context (defaults to True) in the teacher to include speaker in the context, instead of in the label.

Before
    text: A: Hello!
    label: B: Hi!

After
    text: A: Hello!
          B:
    label: Hi!

• Add a command line option --silence-token-dropout (defaults to 1) that determines the dropout rate for including __SILENCE__ token in the generated training examples.

Testing steps
pytest test.py

Data Display

parlai dd --task friends

[mojtaba-komeili]

Looks good. Thanks!